The Effect of Nitrogen and Carbon Supply on the Development of Soil Organism Populations and Pine Seedlings: A Microcosm Experiment

Abstract

Series of pots containing Scots pine [Pinus sylvestris] seedlings and a humus-sand mixture were watered twice weekly for 398 days with different nutrient solutions (control treatment with complete plant nutrient solution less N; glucose addition; N addition; glucose and N addition). Analyses were made of organic and N contents in the substrate, plant weights and N contents, N fixation and respiration rates, abundance, biomass and in some cases species composition of different soil organism populations. The growth rate of pine seedlings was lowest in series supplied with glucose, which is most easily explained by a deficiency of N due to microbial immobilization. The fungi and yeast were stimulated by addition of an easily available C source whereas the bacteria needed both N and C to maintain high biomass. A positive correlation between fungal feeding soil organisms and amount of fungal mycelium was found, while a more complex situation prevailed with regard to bacterial numbers and bacterial feeding nematodes. The systems are evaluated in relation to the different treatments and compared with the field situation.